Romans 1:17

For in the gospel a righteousness from God is revealed, a righteousness that is by faith from first to last, just as it is written: "The righteous will live by faith."

(v17, NIV)

Why does Paul reference Habakkuk? Well, in Habakkuk, God has just revealed that he will unleash the Chaldeans and they're going to come and destroy and take what's not theirs. This wave of wrath is upon them at God's command and they cannot do anything about it to escape it. Is there any hope for them? Habakkuk can't understand how or why God would allow this. In response to his second complaint, God mentions this line that Paul quotes. The righteous will live by faith is what Paul quotes. Apparently, scholars argue whether Paul misquotes this verse or not. Some say he is too loose with the text, making it mean what he wants it to mean. Regardless of whether he is or not, the fact remains that we must live by faith. In Habakkuk, the Chaldeans are coming upon them and they cannot escape it. This is initiated by God and thus it is God approved. In the same way, we are under God's righteous wrath. There is no way for us to escape it, for all have sinned and fall short of his glory. There is nothing that we can do to save ourselves by our own will or work. The only thing we can do and the only thing those in the time of Habakkuk could do is to live by faith. We must live and trust in God because we can do nothing in our own strength.